Separation membranes have various potential industrial applications including natural gas separation and CO2 capture from power plant flue gases. Membrane-based gas separations have a growing market share due to low energy requirements and facile scale-up of the separation unit. Currently, gas separation applications may involve the use of porous polymeric or inorganic membranes. Polymeric membranes used for gas separation applications may be fabricated in a hollow fiber form. Hollow fiber modules have high surface area/volume ratio, typically in the range of 5,000-10,000 m2/m3, which is an important design consideration for commercial large-scale processes. While polymeric hollow fibers may be adequate for some separation processes, the gas separation performance of polymeric materials may be limited by their chemical composition and structure.
Despite concentrated efforts to tailor polymer structure to improve separation properties, current polymeric membrane materials have seemingly reached a limit in the trade-off between productivity and selectivity. For example, many polyimide and polyetherimide glassy polymers such as ULTEM® 1000 have much higher intrinsic CO2/CH4 selectivities (˜30 at 50° C. and 690 kPa (100 psig) pure gas tests) than those of polymers such as cellulose acetate (˜22), which are more attractive for practical gas separation applications. These polyimide and polyetherimide glassy polymers, however, do not have permeabilities attractive for commercialization compared to current commercial cellulose acetate membrane products. Furthermore, such polymers are prone to plasticize (i.e., swell) upon exposure to high-pressure gases such as CO2, thereby making them unselective.
On the other hand, some inorganic membranes, such as SAPO-34 and DDR zeolite membranes and carbon molecular sieve membranes, offer much higher permeability and selectivity than polymeric membranes for separations. An additional benefit is that these materials do not plasticize. However, their processing routes are currently too expensive and difficult for large-scale manufacture.
Therefore, it remains highly desirable to provide an alternate cost-effective membrane with improved separation properties compared to the polymer membranes. In particular, a long-standing goal has been to produce a selective inorganic membrane on a highly scalable and economical platform (such as a polymeric hollow fiber).
To make fluid separation membranes more competitive with other separation processes, such as distillation, adsorption and cryogenic separations, there is a need to develop novel membranes with at least one of the following properties:
a) Gas separation selectivity comparable or superior to polymeric membranes, and higher throughput than polymeric membranes;
b) High membrane surface area/volume (e.g., hollow fiber membrane module); and
c) Facile scale-up for commercial separation processes.